1. Field of the Invention
This invention relates to organic electroluminescent devices having high luminous efficiency.
2. Description of the Prior Art
Organic electroluminescent devices are self-luminous devices based on the principle that, when an electric field is applied, a fluorescent material emits light owing to the energy of the recombination of positive holes injected from an anode and electrons injected from a cathode. Since low-voltage driven organic electroluminescent devices of the laminated structure type were reported by C. W. Tang et al. (e.g., C. W. Tang and S. A. VanSlyke, Applied Physics Letters, Vol. 51, p. 913, 1987), active investigations on organic electroluminescent devices using organic materials as components have been carried on. Tang et al. used tris(8-quinolinol)-aluminum for the luminescent layer and a triphenyldiamine derivative for the hole transport layer. Advantages of the laminated structure are such that the efficiency of the injection of positive holes into the luminescent layer can be enhanced, the efficiency of the formation of excitons by recombination can be enhanced by blocking electrons injecting from the cathode, and the excitons formed in the luminescent layer can be confined. As can be seen from these examples, the well-known structures of organic electroluminescent devices include, for example, a two-layer type consisting of a hole transport (or injection) layer and an electron-transporting luminescent layer, and a triple-layered type consisting of a hole transport (or injection) layer, a luminescent layer and an electron transport (or injection) layer. In these devices of the laminated structure type, various attempts have been made to modify the device structure or their fabrication method and thereby enhance the efficiency of the recombination of injected positive holes and electrons.
However, in organic electroluminescent devices, the probability of singlet formation during carrier recombination is limited owing to its dependence on spin statistics. Consequently, there is an upper limit to the probability of light emission. This upper limit is known to have a value of about 25%. Moreover, in organic electroluminescent devices, light having an exit angle greater than the critical angle undergoes total reflection owing to the influence of the refractive index of the luminescent material, and cannot be taken out of the device as shown in FIG. 1. Consequently, on the assumption that the luminescent material has a refractive index of 1.6, only 20% of the total light produced can be effectively utilized. When the probability of singlet formation is also taken into consideration, energy conversion efficiency is inevitably limited to as low as about 5% (Tetsuo Tsutsui, xe2x80x9cPresent State and Trend of Organic Electroluminescentxe2x80x9d, The Display Monthly, Vol. 1, No. 3, p. 11, September, 1995). In organic electroluminescent devices in which the probability of light emission is severely limited, low light output efficiency would cause a fatal reduction in efficiency.
The method of improving light output efficiency has conventionally been investigated in light-emitting devices having a similar structure, such as inorganic electroluminescent devices. For example, there have been proposed a method for enhancing efficiency by imparting light-condensing properties to the substrate (Japanese Patent Laid-Open No. 314795/""88) and a method for enhancing efficiency by forming reflecting surfaces on the sides or other parts of the device (Japanese Patent Laid-Open No. 220394/""89). These methods are effective for devices having a large light emission area. However, for devices having a minute picture element area, such as dot matrix displays, it is difficult to fabricate lenses for providing light-condensing properties or form lateral reflecting surfaces or the like. Moreover, since the luminescent layer of an organic electroluminescent device has a thickness of several micrometers or less, it is difficult to make the device tapered and form reflecting mirrors on the sides thereof according to current fine machining techniques. Even if it is possible, a considerable increase in cost will be caused. Furthermore, a method for forming an antireflection film by interposing a flat layer having an intermediate refractive index between the glass substrate and the luminescent layer is also known (Japanese Patent Laid-Open No. 172691/""87). This method is effective in improving light output efficiency in the forward direction, but cannot prevent total reflection. Consequently, this method is effective for inorganic electroluminescent devices having a high refractive index, but fails to produce a remarkable efficiency-improving effect on organic electroluminescent devices using a luminescent material having a relatively low refractive index.
Accordingly, the conventional light output method used for organic electroluminescent devices is still unsatisfactory, and the development of a new light output method is essential for the purpose of enhancing the efficiency of organic electroluminescent devices.
Japanese Patent Laid-open No. 83688/96 discloses an organic EL device having a light scattering part on an outside surface of the element. Japanese Patent Laid-open No. 115667/97 discloses an EL device having a light reflecting structure which reflects light from the light emitting surface. Japanese Utility-model Laid-open No. 54184/88 discloses an EL device having micro lense film on the EL element.
These three publications neither teach nor suggest the present organic EL device having a diffraction grating or zone plate as a constituent element.
An object of the present invention is to improve light output efficiency in organic electroluminescent devices and thereby provide organic electroluminescent devices having higher efficiency.
In order to accomplish the above objects, the present invention provides a EL device which has the following feature.
(1) In an organic electroluminescent device having one or more organic layers including a luminescent layer between an anode and a cathode, the device additionally includes a diffraction grating or zone plate as a constituent element.
In preferred embodiments, the present invention also has the following features.
(2) In the device described above in (1), the anode and the cathode form the same picture elements, one of these electrodes is an electrode reflecting visible light, and the diffraction grating or zone plate is formed in this reflecting electrode.
(3) In the device described above in (2), the device has a structure in which the diffraction grating or zone plate, the reflecting electrode, the organic layers and the transparent electrode are formed on a substrate in the order mentioned.
(4) In the device described above in (1), the anode and the cathode form the same picture elements, one of these electrodes is an electrode reflecting visible light, and the diffraction grating or zone plate is formed in the electrode opposite to the reflecting electrode.
(5) In the device described above in (4), the device has a structure in which the diffraction grating or zone plate, the transparent electrode, the organic layers and the reflecting electrode are formed on a transparent substrate in the order mentioned.
(6) In the device described above in (4) or (5), the diffraction grating or zone plate has no light-intercepting part.
(7) In the device described above in any of (1) to (6), the diffraction grating has a two-dimensional periodic configuration.
As described above, the present invention relates to an organic electroluminescent device having one or more organic thin-film layers including a luminescent layer between an anode and a cathode, the device additionally includes a diffraction grating or zone plate as a constituent element. This diffraction grating or zone plate may be either of the reflection type or the transmission type. In the case of a diffraction grating or zone plate of the transmission type, not only an amplitude grating formed by providing it with light-intercepting parts can be used, but also a phase grating formed by modulating the thickness of a layer having a different refractive index may be used to further enhance light output efficiency. Moreover, in the case of a diffraction grating, a grating having a two-dimensional periodic configuration may be used. Thus, as compared with a conventional diffraction grating consisting of a plurality of stripes, light output in a direction parallel to the stripes can be improved.